1. Field of the Invention
The present invention relates to a sector protection circuit for a flash memory device. In particular, the present invention relates to a sector protection circuit for a flash memory device which enables a cell for sector protection to be erased or programmed along with a cell for normal sector thereby to compensate for data loss against the cell for sector protection whenever a normal chip erasure operation or a programming operation is performed.
2. Description of the Prior Art
Generally, cell arrays are consisted of a number of memory blocks 80 through 110, as shown in FIG. 1. Word lines W/L0! through W/LN! of cell arrays 120 are selected by a word line decoder 70, to which a word line voltage generator 60 is connected. Bit lines B/L0! through B/LN! of the cell arrays are selected by a bit line multiplexer 50. In other words, transistors Q1 through QN within the bit line multiplexer 50 are selectively turned on in response to control signals from logic blocks 130, and thereby voltage from a bit line voltage generator 40 is supplied to the memory cells through the turned-on transistor.
A number of protection blocks 10 through 30 are connected to the logic blocks 130 via inverters so that they can perform a programming or erasure operation only for the remaining blocks after once programing any one of the number of memory blocks. For example, in order to protect the first memory block 80, the transistor Q1 is turned off. At this time, signals to control the transistors Q1 through Qn are generated by the logic blocks 130.
FIG. 2 is a detailed circuit diagram of a protection block shown in FIG. 1. A sector protection circuit is required to protect a specific block of a flash memory device against a programming operation or an erasure operation. A sector protection function is performed by programming or erasing a cell S1 for sector protection. The cell S1 for sector protection is programmed or erased in response to output signal of the multiplexer M1. As shown in FIG. 3, when a protect signal PROTECT is a logic 1, an unprotect signal UNPROTECT is a logic 0 and a first control signal (PROGRAM+ERASE) is a logic 0, a program gate of the cell S1 for sector protection is supplied with a positive high voltage. As a second control signal (PROTECT+UNPROTECT) is a logic 0, a PMOS transistor P2 is turned on while a PMOS transistor P1 and a NMOS transistor N1 are turned off. Therefore, the drain of the cell S1 for sector protection is supplied with a supply voltage Vdd, and thus the cell S1 for sector protection is programmed. When the protect signal PROTECT is a logic 0, the unprotect signal UNPROTECT is a logic 1 and the first control signal (PROGRAM+ERASE) is a logic 0, the program gate of the cell S1 for sector protection is supplied with a negative high voltage. As the second control signal (PROTECT+UNPROTECT) is a logic 0, the PMOS transistor P2 is turned on while the PMOS transistor P1 and the NMOS transistor N1 are turned off. Therefore, the drain of the cell S1 for sector protection is supplied with a supply voltage Vdd, and thus data stored at the cell S1 for sector protection is erased. In case that the flash memory device is programmed or erased, the protect signal PROTECT, the unprotect signal UNPROTECT, and the second control signal (PROTECT+UNPROTECT) is a logic 0 while the first control signal (PROGRAM+ERASE) is a logic 1, the PMOS transistor P2 is turned off while the PMOS transistor P1 and the NMOS transistor N1 are turned on. Therefore, the program gate of the cell S1 for sector protection is supplied with a read-out voltage. As a result, data stored at the cell S1 for sector protection are output to an output terminal PROTOUT via inverters I1 and I2. This data protects a specific sector. In this conventional protection circuit, there is a problem that the protect cell could not protect the sector for which protection is required regardless of lapse of time in case that the initially programmed data are not kept in time. There is also another problem that a specified sector could not be protected when data stored at the cell for sector protection are lost by means of stress.